Conveyor for transporting powder, and a method for conveying powder

ABSTRACT

A conveyor ( 10 ) for transporting powder from an inlet point ( 14 ) to at least one discharge point ( 20 ) comprises a fluidized bed transport space ( 12 ) and a fluidization gas supply space ( 16 ), the fluidized bed transport space ( 12 ) being separated from the fluidization gas supply space ( 16 ) by a gas permeable wall ( 18 ); a gas outlet ( 22 ) for removing fluidization gas from the transport duct ( 12 ); means ( 24 ) for separating dust from the removed fluidization gas; and means ( 26 ) for returning the separated dust to the powder proximate the discharge point ( 20 ). In a preferred embodiment, the separated fines dust is returned to, and homogenized into, the powder in a lower portion of a cyclone, which is located at the discharge point.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/059,031 filed Jun. 5, 2008, which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method for conveying powder in afluidized bed transport duct, from an inlet point to at least onedischarge point. The invention also relates to a conveyor fortransporting powder.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,016,053 discloses a system for distributing alumina tosmelting pots. The system is based on air-activated gravity conveyors,i.e. fluidized bed conveyors, in which the alumina is fluidized by avertical, upwardly directed air stream. Gravity thereby makes thealumina flow like a liquid, via fluidized bed transport ducts, to thesmelting pots.

The air used for fluidizing the alumina in the conveyors is allowed toexit from the transport ducts, and is transported, together with thereduction process flue gases from the smelting pots, to a gas cleaningplant, which generally comprises filters, e.g. of bag filter type.

The gas cleaning plant consumes a lot of energy, partly because of thehigh pressure difference required to transport the dust laden gasthrough the bag filters.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve, or at least mitigate,parts or all of the above mentioned problems. To this end, there isprovided a method for conveying powder in a fluidized bed transportduct, from an inlet point to at least one discharge point, comprising

feeding powder into the transport duct at the inlet point;

supplying gas to the transport duct, so as to fluidize the powder in thetransport duct;

removing gas from the transport duct;

separating dust from the removed gas; and

returning the separated dust to the powder proximate the dischargepoint.

By returning the separated dust to the powder proximate said dischargepoint, re-entrainment of the dust in the fluidization gas along thetransport duct is avoided. The dust particles separated from the removedgas is normally the fines, i.e., fines dust particles having a particlesize which is smaller than the average particle size of the powder beingtransported in the transport duct. The fines tend to become entrainedfrom the body of the transported powder under the action of thefluidization gas. Separating the dust, i.e. the fines, from the gasremoved from the transport duct, and returning the separated dust, i.e.,the fines, to the bulk of the powder reduces the accumulation of finesin the removed fluidization gas and in the gas cleaning system, which inturn reduces the pressure drop over the filters in the gas cleaningplant.

In a preferred embodiment, the method further comprises fluidizing thepowder proximate said discharge point, such that the returned separateddust is mixed into the powder, and such that the powder is homogenized.Homogenizing the powder increases the reliability and predictability ofany downstream process making use of the powder, for example thesmelting of the powder in an alumina reduction cell.

Preferably, the gas is removed from the transport duct adjacent to thedischarge point. In this manner, it is easier to maintain a uniformdistribution of particle sizes in the powder over time. This isparticularly a benefit when the flow rate of powder through thetransport duct varies as a function of time. Furthermore, it makes iteasier to maintain a uniform distribution of particle sizes in thepowder in distribution systems having multiple discharge points. Evenfurther, air streams in the upper part of the fluidization bed transportduct will be directed towards the discharge point, which may speed uppowder transport.

In one embodiment, the dust is separated from the gas in a cyclone. Acyclone offers a dust return rate to the discharge point that isrelatively constant over time, since it requires very little periodiccleaning. This leads to a more predictable control of any downstreamprocesses, e.g reduction of alumina in a reduction cell. Furthermore, acyclone is inexpensive, particularly simple to maintain, and makes itpossible to return and mix the separated dust, i.e., the fines, into thepowder within one single device. Preferably, the separated dust is mixedback into the powder in a mixing region in a lower portion of thecyclone. This is a particularly compact and efficient embodiment. Evenmore preferred, the mixing region comprises a fluidized bed; in thismanner, a particularly efficient mixing and homogenization of the powderis achievable.

Preferably, the powder is conveyed from the transport duct to thedischarge point via the cyclone. This minimizes the re-entrainment ofthe separated dust, i.e., the fines, in the fluidization gas.

According to another aspect of the invention, there is provided aconveyor for transporting powder, such as alumina powder, from an inletpoint to at least one discharge point, the conveyor comprising afluidized bed transport duct and a fluidization gas supply space, thetransport duct being separated from the fluidization gas supply space bya gas permeable wall; a gas outlet for removing fluidization gas fromthe transport duct; means for separating dust from the removedfluidization gas; and means for returning the separated dust to thepowder proximate the discharge point. A conveyor of this type reducesthe accumulation of small dust particles, i.e., fines, in the removedfluidization gas supply and in the gas cleaning system, which in turnreduces the pressure drop over the filters in the gas cleaning plant.

In a preferred embodiment, said means for separating dust from theremoved fluidization gas comprises a cyclone, said cyclone having aninlet for dust laden gas; a first outlet for dust; and a second outletfor de-dusted gas, said inlet for dust laden gas being connected to saidtransport duct. A cyclone offers a dust return rate to the dischargepoint that is relatively constant over time, since it requires verylittle periodic cleaning. This leads to a more predictable control ofany downstream processes, e.g. reduction of alumina in a reduction cell.Furthermore, a cyclone is inexpensive, particularly simple to maintain,and makes it possible to return and mix the removed dust, i.e., thefines, into the powder within one single device.

Preferably, said inlet for dust laden gas is connected to said transportduct adjacent to the discharge point. In this manner, it is easier tomaintain a uniform distribution of particle sizes in the powder overtime. This is particularly a benefit when the flow rate of powderthrough the transport duct varies as a function of time. Furthermore, itmakes it easier to maintain a uniform distribution of particle sizes inthe powder in distribution systems having multiple discharge points.Even further, air streams in the upper part of the fluidization bedtransport duct will be directed towards the discharge point, which mayspeed up powder transport.

Preferably, a lower portion of the cyclone communicates with thetransport duct, so as to allow a transfer of powder, such a s aluminapowder, between the transport duct and the cyclone. This is aparticularly compact arrangement for returning separated fines dust intothe powder.

Preferably, said cyclone comprises a gas permeable wall, which separatesthe cyclone from a fluidization gas supply space, so as to allowfluidization of powder in said cyclone. This embodiment is particularlyefficient for mixing and homogenizing the powder.

Preferably, said transport duct is connected to the discharge point viaa lower portion of said cyclone. This minimizes the re-entrainment offines in the fluidization gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of thepresent invention, will be better understood through the followingillustrative and non-limiting detailed description of a preferredembodiment of the present invention, with reference to the appendeddrawings, wherein:

FIG. 1 is a diagrammatic cross-sectional view of a first embodiment of afluidized bed conveyor for transporting powder;

FIG. 2 is a diagrammatic view in perspective of a second embodiment of afluidized bed conveyor for transporting powder;

FIG. 3 is a diagrammatic side view, in cross-section, of a fluidized bedconveyor for transporting powder; and

FIG. 4 is a diagrammatic view, as seen in the section IV-IV, of theconveyor of FIG. 3.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Aluminium is often produced using the Hall-Héroult process forelectrolytic reduction of alumina, i.e. aluminium oxide, to aluminium.The process takes place in reduction cells, or smelting pots, in thepresence of fluorine compounds. Often, aluminium production plantscomprise large-scale distribution systems, capable of transportingpulverous aluminium oxide across distances of several hundreds ofmeters, from a centrally located alumina dispatch terminal, anddistributing it to several hundreds of reduction cells.

Flue gases from reduction cells contain hydrogen fluoride and otheraggressive components, and need to be cleaned in a scrubber. In order tore-use the fluorine, which is essential to the reduction process, theflue gases are scrubbed using primary, i.e. virgin or crude, alumina asa dry adsorbent in the scrubber. A dust collecting facility, whichgenerally consists of a bag filter plant, separates dust from thescrubbed gas, and returns the dust to the alumina in the scrubber. Thesecondary alumina, i.e. the spent alumina that has been used foradsorbing hydrogen fluorine in the scrubber, and that comprises thereturned dust from the dust collecting facility, is then distributed tothe smelting pots for reduction to aluminium, thereby returning thefluorine to the process. The scrubber and the bag filter plant arelocated adjacent to each other, at a central location close to whereprimary alumina arrives at the aluminium production plant, e.g. by truckor by ship. In this manner, the primary alumina may be used in thescrubber, for cleaning the reduction process flue gases, before it isdistributed to the smelting pots via the distribution system.

In a fluidized bed alumina distribution system, also the spentfluidization air that has been used for fluidizing the alumina powder inthe conveyors is transported, together with the reduction process fluegases from the smelting pots, to the gas cleaning plant that containsthe scrubber and the bag filters. In this manner, the spent fluidizationair may be cleaned from any dust particles entrained with it, before thespent fluidization air is discharged into the ambient.

The primary alumina powder, i.e. the aluminium oxide that has not yetbeen spent in the scrubber, is a particulate matter, which comprisesalumina particles ranging from relatively coarse particles, typicallyhaving a diameter of a few mm, to very fine particles of only a fewmicrons or less. A typical primary alumina may, for example, consistmainly of particles in the range from 5 to 200 μm, with only a smallfraction of large lumps up to several mm in size, and a small fractionof very small particles below 5 μm. The secondary alumina also comprisesvery fine fume particles, collected from the flue gas in the scrubber.Those fine fume particles, which may have diameters well below amicrometer, contain a relatively high level of fluorine compounds, andit is therefore desired that they be returned to the reduction process.

The spent fluidization air leaving the fluidized bed conveyor is ladenwith fine dust particles, hereinafter called fines, which may comprisefine alumina particles as well as fume particles. This air comprisingfine dust particles, i.e., fines, which comprises a relative high levelof fluorine compounds, is vented into gas ducts, which convey theextracted flue gas from the reduction cells to the gas cleaning plant.In the gas cleaning plant, the fines following the spent fluidizationair is captured, and together with the captured fumes of the flue gas,it is returned with the secondary alumina back to the transportation anddistribution system. In the distribution system, a significant portionof the fines will once again be entrained with the fluidization air, andtransported to the gas cleaning plant. Fines hence tends to accumulatein the gas cleaning and secondary alumina distribution systems. Thisaccumulation of fines in the systems tends to increase the pressure dropover the gas cleaning plant, thereby increasing the power required totransport the gas through the filters, since the filter bags will beclogged with fines. It may also lead to increased scaling, i.e. depositsof hard dust, in the gas cleaning system, and make the whole secondaryalumina handling and transportation system, its operation, and itsservice a very dusty affair in general.

FIG. 1 schematically shows a first embodiment of a fluidized bedconveyor for transporting powder. The conveyor 10 comprises an uppertransport space 12, which is adapted to receive pulverulent material atan inlet point 14. The powder feed direction into the transport space 12at the inlet point 14 is indicated with an arrow A, and a level ofpowder is illustrated as a hatched area. The conveyor 10 furthercomprises a lower fluidization gas supply space 16, which is separatedfrom the upper powder transport space 12 by a gas permeable wall 18. Thelower fluidization gas supply space 16 is adapted to receive a flow ofgas from a gas supply (not shown), e.g. a ventilation duct, a fan, acompressor, or a container for compressed gas.

Gas is fed into the fluidization gas supply space 16 in the directionindicated by arrow B, and is allowed to enter the powder transport space12 via the gas permeable wall 18, such that it fluidizes the powderpresent in the powder transport space 12 with a vertical gas flow.Examples of suitable gas permeable walls are, e.g., textile fabrics,metal filament webs, perforated plastic or metal sheets, sintered metalsheets, or the like.

The fluidized powder will, under the force of gravity, float slowlyalong the transport space 12 to a discharge point 20, where powder isdischarged from the conveyor 10 into a piece of downstream equipment(not shown).

Spent fluidization gas, i.e. gas that has passed from the fluidizationgas supply space 16 and through the powder in the transport space 12, isvented out via spent fluidization gas outlets 22, which are arranged inan upper portion of the transport space 12. The spent fluidization gaswill also entrain dust, mainly the smallest particles, i.e., the fines,from the powder inside the transport space 12, thereby removing parts ofthe smallest particle fractions from the transported powder. This meansthat the fraction of smaller particles in the transported powder willdecrease with the transport distance from the powder inlet 14.

The fines laden spent fluidization gas is directed to a dust separatingmeans 24, e.g. a cyclone or a filter, in which the fines dust isseparated from the spent fluidization gas. The spent fluidization gasmay thereafter be returned to the fluidization gas supply (not shown),be even further cleaned in additional gas cleaning plants, or bedischarged elsewhere.

The separated fines, on the other hand, is returned to the transportedpowder via means 26, located proximate the discharge point 20, forreturning the separated fines to the powder. This means that at thedischarge point, the fraction of smaller particles in the transportedpowder will be restored.

The means 26 for returning the separated fines to the powder may be,e.g., a gravity fed pipe, a blower, a conveyor, an outlet of the dustseparating means 24, a mixing device for mixing the separated fines withthe powder, or any other means suitable for returning the separatedfines to the powder. Preferably, the means 26 for returning theseparated fines is located in connection with the discharge point 20.More preferably, the distance from the discharge point 20 to the means26 for returning the separated fines to the transported powder is lessthan 20% of the distance from the powder inlet 14 to the discharge point20, and still more preferably, the means 26 for returning the fines tothe powder is located less than 1 m from the discharge point 20. Byreturning the fines relatively near the discharge point, there-entrainment of fines with the fluidization gas, as the powder istransported from the means 26 for returning the fines to the powder tothe discharge point 20, is reduced. In a preferred embodiment, theseparated fines is returned to the powder at a location downstream ofany fluidized bed portion of the transport duct 12, as is illustrated inFIG. 1, such that no fines is re-entrained with the fluidization gas inthe transport duct 12.

FIG. 2 illustrates a second embodiment of a fluidized bed powderconveyor. The conveyor 110, which is particularly well suited forconveying alumina powder, comprises a powder transport duct 112, and afluidization air duct 116. The two ducts are separated by a fabricmembrane 118, which is designed so as to allow air to penetrate thefabric membrane 118 from the fluidization air duct 116 to the powdertransport duct 112. The conveyor 110 extends in an essentiallyhorizontal direction, from a powder inlet point 114 to a plurality ofpowder discharge points 120, of which two are shown. Each dischargepoint 120 is a point of delivery of the powder to another device, suchas a smelting pot, a hopper, a silo, another conveyor, or the like.

At each powder discharge point 120, spent fluidization air is allowed toexit the powder transport duct 112 via a spent fluidization air duct122. The spent fluidisation air is forwarded through the spentfluidization air ducts 122 to cyclones 130, which are also located oneat each of the powder discharge points 120. In each cyclone 130, dust,including the fines, is separated from the spent fluidization air, andthe dust is returned to the transported alumina powder at the respectivedischarge point 120. The cleaned spent fluidization gas leaves therespective cyclone 130 via a respective duct 140 and is transported tothe gas cleaning plant, not shown, together with gases from the smeltingpots, via a central return duct 141.

The cross-sectional view of FIG. 3 more clearly illustrates the functionof the conveyor 110. Secondary alumina powder (hatched) from the gascleaning plant is discharged into a feed hopper 132, from which it isfed to an inlet point 114 of the fluidized bed powder conveyor 110 via arotary feeder 134. From the inlet point 114, the powder is conveyed viathe transport duct 112 to a plurality of discharge points 120. Air fromthe fluidization air duct 116 keeps the powder fluidized along thetransport duct 112, and is vented out from the transport duct 112 viathe spent fluidization air ducts 122. At each discharge point 120, thespent fluidization air is separated from any fines, which may have beenentrained with the air from the fluidized powder. The separation is madein an upper portion 136 of each cyclone 130.

The cross-sectional view of FIG. 4 illustrates a discharge point 120 anda cyclone 130 in more detail. The spent fluidization air duct 122 isconnected tangentially to the upper portion 136 of the cyclone 130, suchthat the spent fluidization air entering the cyclone 130 will form avortex in the upper portion 136 of the cyclone 130. Due to centrifugalforces, dust, including most of the fines, will be separated from thespent fluidization air, and gravity will make the dust fall along thecyclone walls to a lower portion 138 of the cyclone 130. The spentfluidization air, now de-dusted, is discharged through a duct 140 forde-dusted air, which is connected to an upper central portion of thecyclone 130. The duct 140 for de-dusted gas is connected to a gascleaning plant (not shown), preferably via the return ducts for fluegases from the smelting process in the reduction cells. In the gascleaning plant, any remaining dust will be removed from the spentfluidization gas.

For efficient separation of dust from the spent fluidization air, thecyclone 130 preferably has an inner diameter within the range of 75-200mm, and more preferred within the range of 100-150 mm, in the portion ofthe cyclone 130 where the separation takes place, i.e., in the upperportion 136.

A lower portion of the transport duct 112 is connected to the lowerportion 138 of the cyclone 130 via an intermediate duct 142. Fluidizedalumina powder in the transport duct 112 is allowed to flow via theintermediate duct 142 into the lower portion 138 of the cyclone 130,where it is fluidized by air from a cyclone fluidization air supplyspace 144 located below the cyclone 130. Preferably, the intermediateduct 142, which is also shown in FIG. 2, has a cross-section of at least1000 mm², and more preferred at least 2000 mm², in order to permit asufficient flow of alumina powder from the transport duct 112 into thecyclone 130.

The lower portion 138 of the cyclone 130 is, as illustrated in FIG. 4,separated from the cyclone fluidization air supply space 144 via an airpermeable membrane 145, which may be similar to the air permeablemembrane 118. The cyclone fluidization air supply space 144 receivesfluidization air from the fluidization air duct 116 via a conduit 146,which is also shown in FIG. 2, and forwards the air to the lower portion138 of the cyclone 130, so as to form a fluidized bed in the lowerportion 138 of the cyclone 130. In the fluidized bed of the lowerportion 138 of the cyclone 130, the dust, including the fines, separatedin the upper portion 136 of the cyclone 130 and falling down into thelower portion 138 of the cyclone 130, is efficiently mixed with thepowder supplied to the lower portion 138 of the cyclone 130 via theintermediate duct 142. The cyclone 130 in this example thereby acts bothas a dust separating means and a means for returning the separated finesto the powder. It is, however, also possible to use separate means forreturning the separated fines into the powder, for example any of themeans 26 for returning the separated fines to the powder describedhereinbefore with reference to FIG. 1.

In the fluidized bed in the lower portion 138 of the cyclone 130, thesecondary alumina will be homogenized with regard to particle sizes,such that any coarse lumps will remain mixed in the alumina flow.

From the lower portion 138 of the cyclone, the fluidized, mixed, andthereby homogenized secondary alumina powder, which now once again has arestored fraction of fines, is discharged at the discharge point 120into a silo 148, which is configured to forward the powder into aluminareduction cells (not shown).

In a preferred embodiment, each of the discharge points 120 is locatedin connection with a respective alumina smelting pot, and morepreferred, each of the discharge points 120 is located less than 5meters upstream its corresponding smelting pot, such that thetransported powder, now being homogenized, will have little chance tosegregate again before arriving at the smelting pot.

The conveyor 110 may also form a part of a larger alumina distributionsystem. Thanks to the homogenization of the secondary alumina at thedischarge point, any trapping or accumulation of coarse aluminaparticles in downstream regions within the distribution system that maybe unfluidized or present an altered fluidization gas flow, such astransportation duct joints or bends, will be reduced. It is alsobeneficial for any downstream powder feeding equipment, as well as forthe efficiency of the alumina smelting process, that the secondaryalumina be homogenous with respect to particle size when it enters thesmelting pots.

The invention has mainly been described above with reference to a fewembodiments. However, as is readily appreciated by a person skilled inthe art, other embodiments than the ones disclosed above are equallypossible within the scope of the invention, as defined by the appendedclaims.

For example, the invention is not limited to cyclones. Also otherseparating means, for example filters, may be used to separate the dust,including the fines, from the spent fluidization gas.

The invention can be used for transporting other pulverous substancesthan alumina, e.g. fly ash, metal powders, coal powder, and various gascleaning by-products.

Furthermore, even though it is preferred, it is not necessary that theseparating means be located adjacent to the discharge point; theseparation may be performed elsewhere, and after the separation thefines may be conveyed to the discharge point for discharge into thepowder.

1.-13. (canceled)
 14. A conveyor for transporting powder including dustfrom an inlet point to at least one discharge point, the conveyorcomprising: a fluidized bed transport duct for receiving powderincluding dust fed at the inlet port; a fluidization gas supply spacefor supplying fluidization gas to the fluidized bed transport duct tofluidize the powder in the fluidized bed transport duct for transportingthe powder to the at least one discharge point downstream of thefluidized bed transport duct; a gas outlet transport duct bypass forremoving fluidization gas from the fluidized bed transport duct; meansfor separating dust from the removed fluidization gas in the gas outlettransport duct bypass, the means for separating dust including an inletfor receiving dust laden gas, a first outlet for collecting the dustfrom the dust laden gas, and a second outlet for collecting de-dustedgas having the dust separated therefrom; and means for returning theseparated dust from the gas outlet transport duct bypass to thefluidized power by the first outlet which is arranged proximate thedischarge point, wherein the means for separating dust mixes theseparated dust back into the powder in a mixing region in a lowerportion of the means for separating dust.
 15. A conveyor according toclaim 14, wherein said means for separating dust from the removedfluidization gas comprises a cyclone, said cyclone including: an inletfor dust laden gas; a first outlet for dust; and a second outlet forde-dusted gas, wherein said inlet for dust laden gas is connected tosaid transport duct.
 16. A conveyor according to claim 15, wherein saidinlet for dust laden gas is connected to said transport duct adjacent tothe discharge point.
 17. A conveyor according to claim 15, wherein alower portion of the cyclone communicates with the transport duct, so asto allow a transfer of powder between the transport duct and thecyclone.
 18. A conveyor according to claim 17, wherein said cyclonecomprises a gas permeable wall, which separates the cyclone from afluidization gas supply space, so as to allow fluidization of powder insaid cyclone.
 19. A conveyor according to claim 17, wherein saidtransport duct is connected to the discharge point via a lower portionof said cyclone.